In respective fields of iron and steel, non-ferrous metal, ceramics and others, non-oxidizing reduction combustion with an air ratio of 0.5 to 0.95 (operating air ratio varies depending on each target furnace) is conventionally carried out in order to reduce an oxidation or generation scale, or improve or reform a material quality in a heat treatment step of a material. For example, in case of a ceramic baking furnace 102 shown in FIG. 16 as a typical example of a prior art non-oxidizing combustion furnace, a plurality of reduction type burners (capable of performing combustion at an air ratio not more than the theoretical air ratio) 101 are set so that a work 103 is heated in this non-oxidizing atmosphere (for example, an ordinary temperature to 1,250° C.), extracted and carried to a next processing step. Although generation of soot is suppressed as much as possible by a well-designed mixing mechanism in the burners 101, generation of CO can not be of course avoided. Thus, usually, CO is burned in an after burner 104 provided in the vicinity of an outlet of the furnace, and exhaust gas is then passed through a recuperator 105 provided at one position. In this recuperator, heat exchange with combustion air is carried out, and the exhaust gas is emitted from a stack 108 at usually 300 to 400° C. Also, there are many facilities which do not have the recuperator 105. It is to be noted that reference numeral 106 in the drawing denotes a blower and 107 designates an exhaust fan.
In such non-oxidizing reduction combustion, fuel which mainly contains gas is used. In case of low air ratio combustion and combustion at a ratio not more than the theoretical air ratio, however, a large amount of soot as well as CO is apt to be generated in combustion gas in regular burners, and it is hard to stably maintain combustion.
Thus, in the non-oxidizing reduction burner, there are made special contrivances such as acceleration of initial mixing of fuel and air for suppressing occurrence of free O2 (remaining O2) or preliminary mixing of a part of air into fuel for increasing stability. For example, as in the burner 101 shown in FIG. 17, there is provided a mixing structure which emits a jet of combustion air led from an air intake 114 from an air nozzle 111 having several tens of small holes circularly arranged around a fuel injection nozzle 113 at an inlet part of a burner time 110 with a strong swiveling action so that initial mixing of fuel gas and combustion air is rapidly performed in a space 112 of the burner time 110. In case of the burner 101 having this mixing structure, smoke is not emitted at up to an air ratio of approximately 0.6. It is to be noted that the reference numeral 115 in the drawing denotes a pilot burner.
However, in the conventional non-oxidizing burner 101 having the mixing mechanism shown in FIG. 17, the energy can not be saved, and occurrence of soot can not be further suppressed. That is, even if smoke is not emitted, the Bacharach smoke number of approximately 3 is presented, and this is an extent that cannot be admitted as release of fume. Further, since such a non-oxidizing burner 101 as shown in FIG. 10 requires a complicated mixing mechanism and the burner tile 110 in order to assure the mixing property, there is a limit in dimension, and the number of burners must be increased when scale-up (increase in a quantity of combustion) is necessary. Even if a plurality of burners are tried to be assembled, they can not be accommodated. Furthermore, since air and fuel are rapidly mixed (turbulent diffusion mixing) immediately after injection and pre-mixed combustion is carried out, a range of stable combustion conditions is narrowed unless a flame stabilizing mechanism is satisfactory. Moreover, since a mixing ratio approximates to the air ratio of 1.0, a flame maximum temperature is heightened and generation of NOx is rapidly increased.
On one hand, although there is carried out an attempt that the regenerative burner technique which is superior in the energy saving, the low NOx property and the uniform temperature distribution characteristic is applied to non-oxidizing reduction atmosphere combustion with the air ratio of 0.5 to 0.95, this is yet to come into practical use.
On the other hand, since the regenerative burner preheats combustion air to 1000° C. or a higher temperature, NOx is apt to be generated. Therefore, injection nozzles for fuel and air are separated from each other by a predetermined distance or a longer distance, and initial mixing is delayed by injecting fuel in parallel to an air jet. At the same time, the gas circulation effect in the furnace is utilized to the fullest by using the high-speed air jet. Therefore, the high-temperature air is diluted with the exhaust gas, and the combustion reaction is effected in the low-oxygen state so that high-temperature areas are not locally formed in the flame, thereby reducing NOx.
In addition, in case of the regenerative burner, its preheated air temperature is higher (700 to 1,000° C.) than that of a regular non-oxidizing burner, and this temperature is close to a soot generation temperature range of an usual hydrocarbon-based fuel. Therefore, the regenerative burner tends to advance generation of stool as compared with the regular burner.
Accordingly, since there is obtained slow combustion that initial mixing is extremely delayed in the non-oxidizing reduction combustion with a ratio less than the theoretical air ratio, soot is necessarily generated. According to experiments by the present inventor, the Bacharach smoke number was approximately 8 to 9, and a large quantity of NOx was generated.
Additionally, occurrence of soot results in occlusion of the regenerator, and there is fear of reduction in performance of the regenerator, increase in pressure losses and increase in frequency of maintenance.
Further, since the high-temperature gas (not less than 1,000° C.) flows backwards into the burner in a short period (an air throat, i.e., an air passage also functions as an exhaust gas passage), it is hard to provide a complicated initial mixing mechanism for fuel and air such as the non-oxidizing burner shown in FIG. 17 due to the thermal restriction and a limit in pressure losses, and use of metal is also restricted. Therefore, such a burner is yet to come into practical use.
As a basic method for restricting generation of NOx in combustion, there are (1) reduction in a flame temperature, (2) reduction in oxygen density, and (3) shortening a staying time. Thus, in general, it is considered that a high-temperature area is generated in the flame when air having high oxygen density is rapidly mixed with fuel, thereby rapidly increasing thermal NOx. Therefore, in order to suppress generation of NOx, fuel or combustion air is injected in two stages to cause so-called thick and thin fuel combustion so that the flame is prevented from reaching a high temperature, or combustion gas is forcibly circulated in the burner by a flowing wake generated by jet flows of air and fuel so that the oxygen density in this area can be lowered, and dilution of fuel is accelerated to lower a flame temperature so that a quantity of NOx to be generated can be decreased.
Further, in recent years, for the purpose of energy saving, there is proposed a regenerative burner which collects heat of combustion exhaust gas by utilizing a regenerator and uses the collected heat for preheating combustion air to be again put into the furnace. In this burner, combustion air itself is preheated to 1,000° C. or a higher temperature and NOx is apt to be generated. Furthermore, it is considered that air having the high oxygen density and fuel are rapidly mixed as mentioned above when air and fuel are caused to collide with each other immediately after injection, and a maximum temperature of the flame is increased, thereby rapidly increasing NOx.
Therefore, there is an attempt that fuel and high-temperature air are injected into the furnace in parallel to each other with a sufficient distance maintained therebetween, rapid initial mixing of fuel and air is suppressed and, at the same time, combustion is started after a sufficient amount of exhaust gas is involved before mixing of fuel and air and the low oxygen density is then obtained.
In case of thick and thin fuel combustion using air having an ordinary temperature, since combustion can not be carried out unless air and fuel are caused to collide with each other immediately after injection and well mixed, a high-temperature area of the flame can not be satisfactorily prevented from occurring by diffusion combustion of air having the high oxygen density and fuel. Moreover, in case of the exhaust gas recirculation combustion, exhaust gas can not be sufficiently fetched into the narrow burner. It can not be said that reduction in NOx is satisfactory in the both cases.
On the other hand, in case of a combustion method utilizing high-temperature air, since a combustible mixing range of air and fuel is increased, a combustion area does not locally exist but is widely diffused, thereby sufficiently suppressing generation of NOx. On the contrary, in order to realize low-speed mixing by injecting fuel and high-temperature air into the furnace in parallel to each other with a sufficient distance maintained therebetween, a very long staying time in the furnace is required. Thus, sufficient combustion can not be performed in a narrow furnace/short furnace, and unburned fuel/CO may be disadvantageously emitted. Accordingly, a furnace having a sufficient length (a sufficient distance between a pair of burners on the combustion side and the exhaust side) is required, and there is a problem that the furnace becomes large in dimension.
Therefore, it is an object of the present invention to provide a non-oxidizing reduction combustion method and a burner apparatus capable of reducing an amount of remaining oxygen as much as possible with less generation of soot. In addition, it is another object of the present invention to provide a non-oxidizing reduction combustion method and a burner apparatus capable of applying a regenerative burner technique which is superior in the energy saving, the low-NOx property or the uniform temperature distribution characteristic to non-oxidizing reduction atmosphere combustion with an air ratio being less than the theoretical air ratio and being 0.5<m<1.0 in particular.
It is still another object of the present invention to provide a low-NOx combustion method and a burner which can obtain a flame temperature whose distribution is flat which is the same as that obtained with a long furnace length and can perform complete combustion without generating CO. Additionally, it is yet another object of the present invention to provide a low-NOx combustion method and a burner structure capable of applying a regenerative burner technique which is superior in the energy saving, the low-NOx property and the uniform temperature distribution characteristic even if a furnace length is short.